The present invention relates generally to air velocity or flow sensors. More specifically, the present invention relates to a structurally improved thermal anemometer-type mass air flow sensor and method for making same.
Air velocity sensors that are commercially available are commonly of the single hot wire or thermistor type and are typically mounted on the end of a long probe for insertion into an air stream. The temperature drop and the associated change in electrical resistance caused by the cooling effect of the air stream is a measure of the airflow velocity. Disadvantages of such an apparatus are for example the limited durability of the device in an automotive environment and an erroneous reading due to deposits of impurities, commonly found in the air flow of an engine, on the hot element.
As a result of these problems, many attempts in the art have been made to produce a flow sensor which utilizes silicon and its semiconductor properties, or a pyroelectric material. These attempts improved the state of the art in some respects, yet remained deficient with respect to many of the characteristics desired in a modern flow sensor. As is known, a very fine platinum or tungsten wire may be supported in the free stream and maintained at a temperature above the air intake temperature of an automobile engine by electronic sensing and feedback circuits. Any change in the air flow alters the cooling effect of the air on the heated wire. An electronic circuit can sense this change in heat transfer rate and change the heating current so as to maintain the temperature of the wire at a set value.
McCarthy, U.S. Pat. No. 4,594,889 (commonly assigned), provides a method of fabricating a mass air flow sensor on a planar silicon substrate. A pair of spaced openings are formed through the silicon substrate so that a relatively elongated, thin wire-like silicon region remains between the openings. After a silicon dioxide coating is formed on this silicon substrate, an elongated metal coating is applied to the silicon dioxide on the wire-like silicon region. The device fabricated thereby may be used as a low cost, high speed sensing element, such as a mass air flow sensor for measuring air flow in an electronic engine control system. However, the McCarthy flow sensor incorporates an elongated, thin silicon member which is susceptible to breakage upon impact of particles moving with the fluid to be measured, such as dust particles travelling with the air flow in the intake of an automobile engine control system.
Similarly, Bohrer et al, U.S. Pat. No. 4,825,693, disclose a semiconductor device which can be used as a flow sensor. The semiconductor body has a "bridge-type" support diaphragm upon which heating and sensing elements are deposited. The "bridge-type" diaphragm is suspended over a depression formed by the etching of a silicon substrate. The "bridge-type" diaphragm is relatively long and thin and is connected to the semiconductor body at one or more edges of the depression. The "bridge-type" diaphragm consists of metal sensing and heating elements disposed between two dielectric layers such as silicon nitride or silicon oxide. Such heating and sensing elements are entirely located on the bridge portion of the "bridge-type" diaphragm and are thus similarly suspended over the depression. However, no supports are provided under the "bridge-type" diaphragm between the edges of the depression. Since the sensing and heating elements are located superposed on the "bridge-type" diaphragm, their sole means of support derives from the diaphragm-edge attachment. Thus, while this structure results in minimized conductive heat losses to the semiconductor body via its minimum contact therewith, those minimum contacts also result in a relatively fragile structure that is easily susceptible to breakage.
In yet another attempt in the art, Gruner, U.S. Pat. No. 4,498,337, discloses a flow sensor having a flat, metal support base or carrier, such as titanium, which supports thin film resistors or sensing elements Interposed between the metal support base and resistors is an electrically insulating layer such as glass. So as to minimize conductive heat loss through the support base to the structure surrounding the center, portions of the sensor edges which are in contact with the surrounding structure are removed. Although Gruner's use of a metal support base may provide the flow sensor with some degree of mechanical strength, the Gruner flow sensor does not appear to have achieved the combination of high mechanical strength and low conductive heat loss as is desired More specifically, while Gruner uses a continuous flat sheet to support the electrical insulating layer and sensing/heating elements, there remains a need for not only an electrical insulating layer, but also a thermal insulating layer so as to preclude any conductive heat loss from the sensing/heating elements. Thus, there remains a need for a flow sensor which achieves a better combination of increased mechanical strength and minimized conductive heat loss.
Air flow sensors must be inexpensive, yet possess a very fast response and be accurate and rugged. These requirements are often conflicting, as evidenced by certain air flow sensors which typically comprise bulky rugged sensing elements resulting in poor response time characteristics because of increased resistivity attributed to the bulky components of larger sensing elements. Conversely, fast responding air flow sensors are typically expensive and have smaller and thus more fragile sensing elements. Moreover, state-of-the-art thermal air flow sensors must be fully inserted into the fluid flow region and, consequently are subject to destruction and/or deterioration by impacting dust, lint, or other debris in the fluid stream. However, if fragile sensing elements of air flow sensors are strengthened by increasing the width thereof, an increase in conductive heat loss from the sensing elements to the support structure is experienced.
Accordingly, there remains a need in the art for an improved thermal anemometer-type mass air flow sensor having structural improvements which render the flow sensor less susceptible to breakage upon impact by particles moving with the fluid to be measured. Furthermore, there remains a need for such a flow sensor which includes these structural improvements without significant increase in conductive heat loss attributed to the materials supporting the sensing/heating elements of the flow sensor. There also remains a need in the art for a method of making the aforementioned flow sensor so that it can readily be mass produced.